﻿# encoding:utf-8
import traceback


a = None
b = 1
if a:
    print('if a')

if a is None:
    print('if a is None')

if b:
    print('if b')

if b is not None:
    print('if b is not None')

if not b is None:
    print('not b is None')

# is not None 不容易误解， not x is None 有可能会误解成 (not x) is None
# 在编译结果上看，是一样的